1. Field of the Invention
The present invention relates to a blood sugar analyzing apparatus having a fixed glucose oxidase membrane (also known as fixed enzyme membrane).
2. Description of the Prior Art
Blood sugar analyzing apparatus of the type described above measures the blood sugar content or glucose concentration in a blood specimen. The apparatus supplies a blood sugar containing specimen to the enzyme membrane sensor to generate a reaction current proportional to the blood sugar content. The blood sugar analyzing apparatus comprises a fixed enzyme membrane elctrode for measuring the reaction current (a reduction current which flows when reducing hydrogen peroxide is generated upon decomposition of the blood sugar by the fixed enzyme membrane), a reaction cell housing the fixed enzyme membrane electrode and a blood specimen or standard solution (hereinafter referred to as blood specimen or the like), and a liquid pump for drawing a buffer liquid for washing the reaction cell in order to keep the fixed enzyme active.
In FIG. 1 a blood sugar measurement electrode or sensor 1 for measuring a reaction current which is proportional to a blood sugar concentration comprises an electrode of platinum and silver having a membrane of glucose oxidase attached to the surface thereof. The blood sugar measurement electrode 1 and a temperature electrode 3 are disposed in a reaction cell 5. A buffer liquid 7 is delivered by a liquid pump 9 into the cell 5 wherein the liquid 7 washes the interior of the cell 5. After the reaction has been finished, the buffer liquid 7 is discharged as a drainage liquid 11. An air pump 13 vibrates a silicon diaphragm 15 to stir a blood specimen which has been introduced through an inlet port 17 into the cell 5, thereby making the concentration of the blood specimen in the cell 5 uniform. A temperature sensor 19 detects the temperature of the cell block. A heater 21 heats the cell block up to about the temperature of a human body (for example, 37.degree. C.), and the blood in the cell 5 is maintained at a temperature equal to a temperature of a human body.
A control device 23 comprised mainly of a microcomputer 25 is connected to a blood sugar analyzing unit 27 through lines 29, 31, 33, 35, 37 and 41. The control device 23 reads out a reaction current from the measurement electrode 1 over the line 29, reads out an amount of temperature compensation with respect to a measured value over the line 29, measures and controls the temperature of the cell block over the lines 35, 37, and controls operation of the liquid pump 9 and the air pump 13 over the lines 33, 41, respectively. The control device 23 is also connected to various switches 43, 45, 47, 49, 51 and 53, a display unit 55, and a printer 56 through lines 63, 65, 67, 69, 71 for controlling input and output devices. Designated at 51 is a specimen number setting switch, 53 is a standard solution value setting switch, 73 and 75 are mode displays for indicating calibration and operation modes, respectively, and switches 43, 45, 47, 49 are mode setting switches for setting for foregoing modes and a paper feed mode for the printer 56, and for setting the switch 51.
The blood sugar analyzing apparatus, while effecting various operations and measurements, goes through several phases such as warming up the apparatus, putting the apparatus on standby, washing the cell, introducing a sample, and washing the cell. More specifically, the sensor is heated up in a "warming-up" phase, the sensor is ready for measurement or awaits a measurement command in "standby", the sensor is washed with the buffer liquid in "washing", the blood sample is introduced or a reaction starting point is detected with transition to a next reaction phase in "sample introduction", and a measured value is converted to a blood sugar concentration and is displayed in "reaction". These phases are indicated by the lighting or flickering of a variety of lamps which let the operator know what phase the apparatus is operating in. To shift the apparatus from one phase to another, it is necessary to detect and judge transient phases or requirements in each phase of the apparatus.
There have heretofore been various problems in detecting phase transitions in the blood sugar analyzers especially when attempting to detect a rising point of a chemical reaction which was started by introducing a blood specimen. It has also been difficult to detect the finishing point of a chemical reaction which converts a reaction current to a corresponding blood sugar concentration.
More specifically, a conventional process of detecting a rising point of a chemical reaction resides in that when a reaction current (which is actually converted to a voltage signal) reaches a predetermined threshold level, the reaction is regarded as being started and is monitored for a given interval of time from that starting point or zero point on. Upon elapse of the given interval of time, for example, 20 seconds, the reaction is regarded as having reached a steady condition, and thereafter a measured value is given by subtracting the threshold value as a base or offset portion from the output of the sensor which increases as the reaction progresses.
The base or offset portion is not necessarily constant and is variable with changes in the condition and temperature in the reaction cell which houses, the enzyme membrane comprising the fixed enzyme membrane and the measuring electrode (the blood sugar analyzing apparatus is particularly affected by temperature changes as it utilizes chemical reactions). If the base or offset portion were rendered constant, the apparatus might erroneously start a measuring operation under some conditions with no blood specimen or the like in the cell, or an excess base portion might be subtracted from a normal reaction quantity, resulting in an incorrect measurement.
According to a process of detecting a reaction ending point, the rising point of the reaction is regarded as a reference point, and the reaction is monitored for a given interval of time from the reference point on, that is, the reaction is assumed to be in equilibrium or finished upon elapse of the given interval of time. Thus, the detection process is dependent only on keeping time.
The fixed enzyme membrane in the blood sugar analyzing apparatus tends to deteriorate and its power to measure blood sugar is reduced with the result that changes in the reaction current will become retarded. As can be seen from FIGS. 2, 2(a) and 2(b) the rapid-response sensor has a steeply rising curve SO.sub.1 after the blood specimen or the like is introduced at a time I, which the deteriorated sensor gives a less steep curve SO.sub.2. Using a deteriorated sensor, simply monitoring a time interval T.sub.1 cannot ascertain whether the reaction is saturated, finished or in progress, and hence fails to sense a correct reaction quantity. With the rapid-response sensor, on the other hand, no measurement is effected until the time interval T.sub.1 elapses regardless of the sensor's being indicative of saturation or termination of the reaction in a shorter period of time. Accordingly, a time interval in which a measurement should be carried out is wasted, and hence the blood sugar analyzing apparatus has a reduced analyzing capability.
When a blood specimen or the like is introduced in the reaction cell in the blood sugar analyzing apparatus, a reaction with the fixed enzyme starts and reaches a steady condition after a certain period of time (a few seconds) has passed, allowing an analyzed value to be available in proportion to the blood sugar concentration in the blood. The time at which the reaction has reached the steady condition is regarded as a time at which the reaction is ended, and the buffer liquid is delivered into the reaction cell to wash the interior thereof including the enzyme membrane so that the next specimen or a standard solution may be introduced into the cell. The reaction cell has conventionally been washed with a constant amount of buffer liquid during a constant period of time. This washing process is disadvantageous in that a blood specimen of a higher blood sugar content in the reaction cell may not be completely flushed away, and a residual amount of blood sugar may be added to the blood sugar content in a next blood specimen, resulting in a reduced degree of measuring accuracy. To cope with this, the cell is required to be washed again. Where a blood specimen of a lower blood sugar concentration is to be washed away, the buffer liquid required for the washing may be supplied in a smaller amount, and therefore use of the constant amount of buffer liquid for the constant period of time is wasteful.